The present invention relates to a fuel assembly and a reactor core, and more particularly to a fuel assembly for use in a boiling water reactor and a core of such a reactor.
A conventional fuel assembly loaded in a boiling water reactor comprises a channel box in the form of a rectangular tube and a fuel bundle housed in the channel box. The fuel bundle comprises upper and lower tie plates respectively fitted to upper and lower portions of the channel box, a plurality of spacers installed in the channel box with intervals therebetween in the axial direction, a number of fuel rods penetrating through the spacers and arrayed in a square lattice pattern with their opposite ends fixed to the tie plates, and at least one water rod.
Recently, raising a degree of burn-up of a fuel assembly has been attempted from the standpoints of prolonging the operating time, effectively utilizing uranium resource, and reducing the amount of spent fuel generated. For achieving a higher degree of burn-up, it is required to increase enrichment of a fuel assembly. With enrichment increasing, however, larger mean energy of neutrons has raised the problem that reactivity change due to void variations may increase, or effective utilization of fissionable material (fuel economy) may be impeded. The increased reactivity change due to void variations not only enlarges an absolute value of the void coefficient and lowers core stability, but also reduces a shutdown margin because of an increase in the hot-cold swing. Such a tendency is dealt with by increasing a moderator proportion (i.e., a ratio of moderator to fuel) in the fuel assembly and reducing mean energy of neutrons (i.e., making the neutron spectrum softer).
In a boiling water reactor, control rods and neutron detecting counters are disposed outside the channel box. Therefore, a gap is defined between fuel assemblies for allowing those units to be inserted therein. Since the gap is filled with saturation water, those fuel rods which are positioned in a peripheral portion of the fuel assembly (i.e., in a region nearer to the gap) and those fuel rods which are positioned in a central portion of the fuel assembly are affected by the saturation water in the gap in different ways. Specifically, in the peripheral portion of the fuel assembly nearer to the gap, the ratio of moderator to fuel is so large as to increase a moderating effect, thus making nuclear fissions in the fuel rods at such a position more active. On the contrary, the fuel rods positioned in the central portion of the fuel assembly are less affected by a moderating effect due to the saturation water in the gap. Thus, the ratio of moderator to fuel, as a factor of determining nuclear characteristics of a fuel assembly, is different depending on the position of the fuel assembly.
There are two methods of raising the ratio of moderator to fuel; i.e., a method of reducing a fuel inventory and a method of increasing a moderator region or moderator density. Practically, these methods are carried out by (1) increasing a boiling water region (e.g., diminishing the number of fuel rods or thinning the diameter of fuel rods), and (2) increasing a non-boiling water region (water rod region or gap water region).
In the fuel assembly prepared by adopting one of the above methods, however, the fuel inventory is reduced in any case, meaning that fuel economy is improved from the aspect of enlarging the ratio of moderator to fuel, but fuel economy is impeded in terms of the fuel inventory. Eventually, an improvement in fuel economy is not achieved. Furthermore, the above-mentioned methods give rise to new problems. With the method (1), the reduced total length of fuel rods increases a linear heat generation rate and decreases a thermal margin. With the method (2), the reduced area of flow paths for a coolant makes a pressure drop larger.
In a conventional fuel assembly, fuel rods are arrayed into a lattice pattern of 8 rows and 8 columns (i.e., 8.times.8). If the number of unit lattices in the fuel rod array is increased to 9.times.9 or 10.times.10, it would be possible to reduce a linear heat generation rate, enlarge a heat conducting area, and increase a thermal margin. Also, as illustrated in FIGS. 3 and 4 of JP, A, 52-50498, it is known to construct a fuel assembly by using partial length fuel rods which have a shorter fuel effective length. With this type fuel assembly, since the flow path area of a two-phase flow (in an upper portion of the core) having a large friction loss is enlarged, the pressure drop can be suppressed without reducing the fuel inventory. Consequently, by adopting the aforesaid methods (1) and (2) in addition to those two approaches, a fuel assembly can be obtained which is suitable for raising a degree of burn-up.
In view of the above, there has been proposed a fuel assembly that fuel rods are arrayed in a lattice pattern of 9.times.9 or 10.times.10 with each fuel rod having a larger outer diameter but the number of fuel rods being increased, the cross-sectional area of a water rod is made larger than that of a unit lattice, and further a plurality of partial length fuel rods are arranged, as disclosed in JP, A, 62-276493, JP, A, 64-31089 and U.S. Pat. No. 5,068,082, for instance.
More specifically, JP, A, 62-276493 discloses a fuel assembly having the increased number of unit lattices in the fuel rod array, in which a number of water rods or large-diameter water rods are arranged, and a plurality of partial length fuel rods are arranged in a row along a diagonal including corners of the lattice array of fuel rods. The partial length fuel rods are denoted by 14 FIGS. 1 and 5.
In FIGS. 1, 7 and 8, etc. of JP, A, 64-31089, there is disclosed a fuel assembly having the increased number of unit lattices in the fuel rod array, in which large-diameter water rods are arranged and one or a plurality of partial length fuel rods P are arranged at one or more corners of the lattice array of fuel rods.
In FIGS. 41 to 56 of U.S. Pat. No. 5,068,082, there are disclosed a fuel assembly having the increased number of unit lattices in the fuel rod array, in which a plurality of partial length fuel rods P are arranged together adjacently to large-diameter water rods. U.S. Pat. No. 5,068,082 also describes a fuel assembly having the increased number of unit lattices in the fuel rod array, in which large-diameter water rods are arranged, and a plurality of partial length fuel rods P are arranged in a row along a line bisecting each side of the lattice array of fuel rods at its outermost layer (e.g., FIGS. 2B, 6 and 10, etc.) or along a diagonal including corners of the lattice array of fuel rods (FIG. 5 and 12, etc.). Furthermore, U.S. Pat. No. 5,068,082 discloses another layout example of the partial length fuel rods P in which the partial length fuel rods P are arranged at each corner and the middle of each side of the lattice array of fuel rods in its outermost layer.